The easiest way that I've found is to use Leandro's
Splasher program to convert the image from a .bmp to a .texture. Then rename the coverted image to COH_LogInScreen_Background.texture and put it in the following location (create each folder inside the previous one until you have the full path).
<CoH folder>\data\texture_library\GUI\CREATION\HybridUI\LoginScreen
Nice pic btw.
Edit: Forgot to mention that you'll need to open the file in a hex editor and replace the path so it looks like this.
TX2texture_library/GUI/CREATION/HybridUI/LoginScreen/COH_LogInScreen_Background.dds DDS